This invention relates generally to hybrid circuits, and more particularly to a hybrid circuit for interfacing a two-wire line and a four-wire line.
In telephone systems and the like it is often necessary to couple a four-wire line to a two-wire line. More specifically, the four-wire line comprises two wires that provide a receive path for signals and two wires that provide a transmit path for signals. The two-wire line provides both a receive path and a transmit path on a common pair of wires. Most frequently, the two-wire line extends from a subscriber's telephone set to a switching office. At the switching office it is common to convert the two-wire bidirectional line to the four-wire (two unidirectional paths) line by use of a hybrid circuit.
Hybrid circuits are well known and the following patents describe but a few of the known techniques in this field: U.S. Pat. No. 2,946,861 dated July 26, 1960 by Li-Yen Chen; U.S. Pat. No. 4,232,293 by H. H. Harris dated Nov. 4, 1980; U.S. Pat. No. 4,322,586 dated Mar. 30, 1982 by G. F. Mein and J. B. Terry; U.S. Pat. No. 4,346,266 dated Aug. 24, 1982 by Brockmann et al; and U.S. Pat. No. 4,346,267 dated Aug. 24, 1982 by E. C. Dijkmans. Attention is respectfully directed to the above listed patents.
Desired objects for a hybrid circuit of this type are to maximize the return loss and to increase the efficiency of the hybrid circuit. Previously this has been accomplished by the use of two R-C (resistor-capacitor) networks on the four-wire side (note: this is depicted in more detail in FIG. 1). Another object is to maximize the transhybrid loss.